Diffusion bonding is a well known and important method of joining metals. When diffusion bonding, it is often desirable to bond the metal in isolated sharply defined areas, and to prevent bonding in other contacting surfaces of the parts to be joined. To prevent bonding in selected areas a coating of a stop-off material is applied. It is known to use powdered boron nitride combined in an inorganic binder such as alumina, magnesium silicate or aluminum phosphate in an aqueous vehicle to form a stop-off for titanium. Coatings using these stop-offs are suitable for brushing or spraying applications, but are unsatisfactory for application by screen printing.
U.S. Pat. No. 2,772,180, to Neel et al, discloses a stop-off material for metals of an aqueous solution of colloidal graphite and calcium carbonate.